1. Field
The Embodiment herein relates to a nonvolatile semiconductor memory device including electrically rewritable memory cells.
2. Description of the Related Art
In recent years, demand for small, large capacity information read/write devices (memory devices) has grown rapidly. Two such devices, NAND flash memory and small HDD (hard disk drive), have shown rapid advances in recording density and thereby come to form a large market. Such circumstances have led to the proposal of a number of ideas for new memory that aim to exceed a limit of recording density.
In this kind of memory, a voltage pulse can be applied to a resistance changing element to repeatedly change the resistance thereof between a low-resistance state and a high-resistance state, thereby storing data by correspondence of these two states to binary data “0” and “1”. Moreover, it is expected multilevel data storage is accomplished to further improve recording density.
However, there arises a problem that, when levels of multilevel storage and recording density are raised, there is an increased probability of incorrect information being written during write or erase of information (there is an increased probability of incorrect switching). Lowering the probability of incorrect switching requires a resistance changing material with a high resistance ratio between the low-resistance state and the high-resistance state.
Under such requirement, various materials have been developed as the material for the resistance changing element, among which a resistance changing material composed mainly of carbon (C) is a leading candidate. For example, there is additionally proposed a nonvolatile memory utilizing a resistance changing material in which titanium (Ti) is embedded within carbon nanotubes to raise the resistance ratio between the low-resistance state and the high-resistance state.
However, there is a problem in a conventional resistance changing material mainly composed of carbon that the resistance value in the low-resistance state becomes too low. If the resistance value of the low-resistance state becomes too low, the problem arises that the cell current increases whereby power consumption increases. As seen above, there is yet to be proposed a resistance changing material that combines suppression of power consumption and reduction in risk of incorrect switching.